The present invention relates to a system for transmitting power of an engine to four wheels of a motor vehicle having a central differential, and more particularly to the system which provides multiple operational ranges of the transmission.
In a conventional four-wheel drive motor vehicle having the central differential, the central differential comprising bevel gears or a planetary gear device is disposed in alignment with an output shaft of an automatic transmission. The automatic transmission is provided with a hydraulic control system and various sensors for detecting driving conditions of the motor vehicle. A fluid operated multiple-disk friction clutch is provided for controlling the central differential. Torque of the clutch is variably controlled in accordance with hydraulic pressure from the hydraulic control system or output signals from a microcomputer for controlling a torque split ratio between front wheels and rear wheels, thereby improving the engine power, driveability, stability at braking, and antilock brake operations.
Japanese Patent Application Laid-Open 63-176728 discloses a four-wheel drive motor vehicle in which a central differential comprising a simple planetary gear device is provided. An output of a transmission is transmitted to a carrier of the simple planetary gear device. The torque is split to front wheels through either of a sun gear or a ring gear and to rear wheels through the other. The torque to the front and rear wheels is unequally split at a ratio determined by the difference between pitch circles of the sun gear and the ring gear. A fluid operated multiple-disk friction clutch as a lock device is provided for controlling the differential operation.
Japanese Patent Publication 53-32466 discloses a power transmitting system for a motor vehicle. The system is provided with two sets of planetary gear devices for providing three forward speeds and one reverse speed of the transmission. A sub-transmitting system comprising complex planetary gear devices is provided for providing a fourth speed of an overdrive.
However, in the conventional systems, it is necessary to provide another sub-transmitting system for providing a further operational range having a larger gear ratio than that of the lowest speed. Consequently, the transmission becomes large in size.